1. Field of the Invention
The present invention relates in general to a system for measuring the distance between a central station and one or plural terminal stations that are mobile with regard to the central station, circuits for performing this measurement being included partly in the central station and partly in each terminal station.
In such a distance measurement system, the central station must be capable, by emitting microwaves towards a terminal station, of determining the variable distance separating it from said terminal station.
2. Description of the Prior Art
According to the prior art, the central station measures the distance between the two stations by emitting microwaves towards the terminal station and by detecting waves reflected by said terminal station. The distance separating the stations is calculated as a function of the product of a delay between the instant of emission and the instant of reception of the reflected waves by the propagation velocity of said waves. For instance, an embodiment of a central station using such a relation includes an emitter emitting a microwave of variable frequency, with the frequency varying in sawtooth manner. At instants t.sub.0, t.sub.1, . . . corresponding to minimum emission frequencies, a microwave "pulse" is emitted by the central station to a target. During the emission and reflection propagation times, the frequency of the microwave produced by the emitter has increased linearly, and at the instant of reception of the reflected wave, it is possible to deduce a distance from a difference in frequency .DELTA.f between the emission frequency and the frequency generated by the emitter at the instant of reception.
Other embodiments exist, but always require complex and expensive circuits, restricting applications to privileged sectors.